Classroom 206 is a classroom in P.S. 118. It's the homeroom of Arnold and the other students, Gerald, Helga, Phoebe, Harold, Sid, Stinky, Rhonda, Lila, Iggy, Peapod Kid, Nadine, Park, Brainy, Joey, Eugene, Curly, Sheena and Lorenzo fourth grade class.
As Hillwood is supposed to be an amalgam of large northwestern cities, including Seattle. The 206 could be a reference to the Seattle area code, a large city in Washington and hometown of Craig Bartlett the creator.